A conic metal pipe is used in various industrial fields such as fields of manufacturing head parts of bullets or missiles, components of vehicles such as aircraft and automobiles, kitchen instruments and heating instruments. Such a conic metal pipe has been conventionally worked into a predetermined configuration through a “Metal spinning” technique.
However, because the “Metal spinning” technique is a metallic working process which is chiefly intended to control configurations of workpieces, it has little relevance to improvement of mechanical properties such as control of fine microstructure. Furthermore, according to the “Metal spinning” technique, because deformation caused by high pressure applied from a metal tool is concentrated to a surface of a metal pipe workpiece, there is a problem in that a great difference between mechanical properties of internal and exterior areas of the worked metal pipe occurs.
A metal material begins to form dislocation cells having small boundary angles when it is subjected to plastic deformation, and crystal grain boundary angles of sub-grains increase and crystal grains increasingly become fine as the amount of plastic deformation increases. Therefore, when metal material is highly deformed and thus crystal grains thereof become ultrafine grains or nanocrystalline grains, the deformed metal material has considerably improved mechanical properties (yield strength, tensile strength, hardness, wear resistance, superplasticity and the like) compared to a metal material which is not deformed. Accordingly, there is a need for a working process for producing a metal material having ultrafine crystal grains/nanocrystalline grains, rather than a conventional working process which is designed to mainly change a shape of the metal material.
The formation of ultrafine crystal grains/nanocrystalline grains is affected by an amount of plastic deformation such as compressive deformation, tensile deformation and shear deformation which a workpiece undergoes. In addition, it is critical to design a metal die which maintains the substantially same shape before and after being subjected to a forming process, such that even when a workpiece undergoes a large amount of deformation the die maintains its shape, so as to allow the execution of repeated processes.
Working processes, which meet the above requirements and have been developed to date, may include Equal Channel Angular pressing (ECAP), High-Pressure Torsion (HPT), Accumulative roll Bonding (ARB), Equal Channel Angular Rolling (ECAR) and the like.
However, a working process which can give a severe plastic deformation to conic metal pipe in accordance with a configuration of the conic metal pipe, has not yet been developed, and thus there is a need for development of this kind of working process.